(a) Field of the Invention
This invention relates to a method and a device for the injection of particles for thermal spraying. More particularly, the present invention relates to a method which enables to inject particles into the plasma flame so as to ensure that substantially all the particles are completely melted and adequately propelled onto a surface intended to be coated. The invention also relates to a device which permits to slow down the speed of the particles and at the same time adjust the flow of carrier gas so as to provide an efficient injection of the particles into the center of the flame.
(b) Description of Prior Art
The plasma spraying process is a process in which materials to be deposited (generally powders) are introduced into a hot gas stream and are thereafter propelled onto the surface of a substrate. The powders consisting of materials to be deposited are heated when they enter the plasma. The heated particles must be molten or in a plastic state when they leave the hot gas effluent. The impacting particles flatten, interlock and overlap one another, securely bonding together and forming a coherent layer of material onto the substrate. When the substrate is properly prepared, it forms an adherent bond with the coating layer.
The material to be sprayed, such as powders, is pneumatically carried from the powder feeder to the plasma gun by means of a gas, generally an inert gas such as argon. This carrier gas serves to convey particles of the material to be sprayed to the plasma gun and also impart kinetic energy to the particles which must be injected as close as possible to the middle of the plasma flame.
One major problem faced by the common practice of plasma spraying is the following. The particles must be injected into the plasma as close as possible to the middle of the flame in order to be completely melted and be adequately propelled onto the substrate. In addition, the flow rate and pressure of the carrier gas must be sufficiently high to carry the powders to the gun but the same should be low enough in order that the particles do not travel transversally through the plasma flame.
Generally, the user of a plasma torch selects flow rate and pressure valves for the carrier gas by a traial-and-error procedure, and experience has shown that most of the time, the particles are not properly injected into the flame. If the gas flow rate and the pressure are too low, either the powders are not carried to the plasma gun or they are deflected by the plasma flame. On the other hand, if the gas flow rate and the pressure are too high, a portion of the particles travel transversely through the plasma flame and are not properly melted and accelerated onto the substrate.
For these reasons, the quality of the coatings produced by plasma spraying as well as the efficiency of the process suffer form the fact that the particles are not properly injected into the plasma flame.
U.S. Pat. No. 4,199,104 issued Apr. 22, 1980 to Johan M. Houben and Japanese 7137081 issued to Kobe Steel Works Ltd. both relate to the release of the carrier prior to injecting the powder into the plasma stream. The Japanese Patent uses a device which consists of a movable sleeve provided with an orifice. U.S. Pat. No. 4,199,104 discloses a perforated or porous wall and a sliding shutter which is impermeable to the gas. Obviously, although these references recognize the problem of properly injecting the particles into the plasma stream, neither of them provide an adequate means for the correct injection of particles into the plasma stream. Even though the momentum of the particles is modified, it is not adequately controlled and the process is not reproducible from one experiment to the other. Furthermore, the use of the devices of both patents is time consuming, a substantial amount of powder would be lost through the orifice or orifices and the porous impermeable membrane can become clogged up or obstructed by the powder.
It is an object of the present invention to provide a method for suitably and correctly injecting the particles into the plasma flame.
It is another object of the present invention to provide a device which permits to slow down the speed of the particles and at the same time adjusts the flow rate of the carrier gas so as to efficiently inject the particles into the center of the flame.
It is another object of the present invention to provide an apparatus which acts to remove a portion of the carrier gas immediately before the latter enters the plasma gun.
It is another object of the present invention to provide for a partial separation of the particles in order to eliminate fine dust and carrier gas fed to the plasma flame and supplied from a powder hopper.
It is another object of the present invention to produce a narrow spray pattern of completely melted particles which results in coatings of better quality and less material waste.